1. Field of the Invention
This invention relates to a paper tractor used in a printer for feeding a recording paper and, more particularly, to the type of paper tractor which employs an endless pin belt having a plurality of pins provided on its peripheral surface which come into engagement with the perforations of the recording paper.
2. Description of the Prior Art
As disclosed in U.S. Pat. Nos. 4129239, 4199091 and 4315585, the typical pin belt type paper tractor known in the art is configured so that it can move in the widthwise direction of the recording paper along two parallel shafts in the form of a drive shaft and a support shaft. A pair of pulleys are rotatably supported on a chassis body, and an endless pin belt is engaged between and around these two pulleys. One of the two pulleys is fitted relatively non-rotatably, but slidably in the axial direction on the drive shaft, and the support shaft passes through the chassis body.
Although the drive shaft and the support shaft must be arranged mutually parallel in parallel to each other, they have inevitably some deviations from the parallel relation owing to errors in manufacture and the like. Hence, in order to assure smooth movement in the axial direction of the tractor even if there exists an error in the parallel relation between the drive shaft and the support shaft, the following configuration is employed. That is, a slider is assembled in the chassis body slidably in the fore and aft direction, and the support shaft passes slidably through this slider.
However, the configuration as described above has the drawbacks that the structure is complicated, the number of parts is increased, the assembly work is troublesome, and hence the manufacturing cost is high.
Further, since back tension is always applied to the recording paper during the printing, if the paper tractor operates at a high speed to feed the recording paper just printed at the time of high-speed printing, the pins of the pin belt become difficult to come off from the perforations of the recording paper, and thus, the endless belt tends to be raised by the recording paper, or the recording paper tends to be torn owing to the timing lag of the pins separating from the recording paper, or the pins sometimes tend to roll up the recording paper while engaging with the perforations to thereby cause jamming.
Each tractor unit can be moved to any position along the drive shaft and the support shaft depending upon the width of the recording paper, and locked at a desired position by means of a clamping mechanism. The clamping mechanism known in the art is movably mounted on the chassis of the tractor unit, and a sleeve with splits is formed on a portion of the slider through which the support shaft passes slidably, and thus, the tractor unit is fixed on the support shaft by tightening the sleeve with splits by means of a fixing lever or fixing screw. Accordingly, additional parts are needed to clamp the track unit at a desired position. In addition, there are the drawbacks that the structure is complicated, the assembly work is troublesome, and the manufacturing cost is high. Furthermore, in clamping the tractor unit, it is necessary to manipulate the fixing lever or fixing screw, hence, if such manipulation is performed carelessly, the tractor unit tends to be displaced during the operation of the printer to thereby cause defective paper feed.